This invention relates to an amplifier for an active terminator connected to an end of a bus line such as SCSI. More particularly, this invention relates to such an amplifier for making high-speed operations of bus lines possible.
It has been known to connect terminal resistors; individually to a bus line for transmitting signals among a plurality of drive units so as, for example, to prevent "ringing." It has also been known to set the resistance of each such terminal resistor so as to match the characteristic impedance of the corresponding line and to output a specified voltage through a terminal resistor to each line. Japanese Patent Publication Tokkai 7-261890 disclosed a system using such an active terminator characterized, as shown in FIG. 5, as having amplifiers each provided to a line of a bus for outputting a specified voltage to the bus through terminal resistors such that the individual lines of the bus are dependably isolated and a high-speed operation of the bus is possible. Explained more in detail, an active terminator 4 is connected at an end of a bus 1 (having a plural n-number of lines BUS1, BUS2, . . . BUSn) to which are connected a plurality of drive units 2, 3, . . . , and the active terminator 4 is provided with n-number of amplifiers 41, 42, . . . 4n each corresponding to a different one of the bus lines BUS1, . . . BUSn, and each adapted to receive a standard voltage V.sub.in from a band gap voltage source (BG) 5 through an input terminal IN and connected through an output terminal OUT to the corresponding one of the bus lines BUS1, . . . BUSn. Each of these active terminator amplifiers 41, 42, . . . 4N comprises a voltage-follower type amplifier receiving the standard voltage V.sub.in from the input terminal IN as its non-inversion input and outputting a voltage V.sub.0 to the output terminal OUT through a resistor R3.
Since a feedback line is provided to each amplifier so as to maintain the voltage V.sub.0 in a matching condition with a specified voltage corresponding to V.sub.in, the output currents from the amplifier vary according to any change in the bus lines BUS1, . . . BUSn due to the operations of the drive units 2, 3, . . . such that appropriate currents corresponding to the differences between the bus line voltage and the specified voltages are supplied by the active terminator amplifiers 41, 42, . . . 4n and hence that the occurrence of unwanted ringing can be prevented.
Such active terminator amplifiers may comprise an ordinary buffer amplifier such as having input transistors to form a differential amplifier circuit and output transistors to be driven by its differential signals, together being assembled like a totem pole. In order to eliminate any interference with other active terminators, however, Japanese Patent Publication Tokkai 7-240642 disclosed an amplifier adapted to selectively output a specified voltage through a terminal resistor in response to an enable signal.
As shown in FIG. 6, such an active terminator amplifier may comprise a pair of input transistors Q1 and Q2 serving as an input differential amplifier circuit and totem pole-shaped output transistors Q8 and Q10. As output transistor Q8 (on the current outlet side) receives a constant current I8 on its base, an idle current is caused to flow through output transistors Q8 and Q10. As one of the input transistors (Q1) receives an input voltage V.sub.in on its base and the other input transistor Q2 receives a feedback voltage V.sub.0 on its base, they generate together a differential signal D1 and, as the output transistor Q10 is driven by a current-amplified differential signal D10, the collector current of the output transistor Q10 changes to a different current from the idle current according to the difference between the voltages V.sub.in and V.sub.0, the difference current which is caused thereby being supplied to any of the bus lines (indicated as BUSi in FIG. 6) through a terminal resistor R3.
Enable signals are adapted to be received through terminal ENB by the gates of transistors QN1 and QN2. When the enable signal is "High," both transistors QN1 and QN2 are switched on, grounding both the line for the differential signal D10 and that for the constant current I8 and thereby leaving the output in a floating condition with high impedance. Thus, they can serve as a buffer amplifier only when the enable signal is "Low."
With active terminator amplifier thus structured being connected to an end of a bus, although it may have a plurality of active terminators connected, the output voltage V.sub.0 through the terminal resistor R3 stays more or less at a specified level as shown in FIG. 7, independent of the variations in voltage on the bus line if enable signals are applied such that only selected ones of them are activated, even when the voltage on a bus line BUSi (I being a dummy index 1-n) changes between High and Low as shown in FIG. 7, say, due to the operation of the drive units. FIG. 8 shows another example of change in waveform.
With the increase in the operating speed of multi-purpose CPUs, for example, there is also an increased demand for high-speed operations of bus lines, and particularly device buses such as SCSI serving to connect peripheral apparatus.
If the speed of voltage switching is simply increased on a bus line, however, the amplifier cannot catch up with sudden changes in the voltage in the bus and phenomena such as the ringing which occur at the end of the transition section of the signal waveform cannot be controlled sufficiently. As a result, disturbances in the waveform become significantly large and it is difficult to maintain the reliability of bus operations. Thus, the operational speed of active terminator amplifiers must be increased for high-speed bus operations. As a direct method for increasing the operating speed of an amplifier, one may think of improving the speed of transistors which form the amplifier. Such a direct method is not practical, however, because the production cost will become unreasonably high. Especially in the case of an amplifier of a voltage follower type used where an output is directly fed back as an input, all of a series of transistors from the input side to the output side must be modified for a higher speed. It should also be kept in mind in this connection that if the operating speed of an element is increased, it tends to become weak against noise.